1. Field of the Invention
This invention relates to a toner for developing electrostatic images. Further, the present invention relates to a detachable unit freely detachable from the main apparatus, comprising a chargeable body and a developing means holding a toner to develope electrostatic images. Further, the present invention relates to an image forming apparatus having a developing means holding the toner for developing electrostatic images. Further, the present invention relates to a facsimile apparatus using in electrophotographic apparatus as the printer.
2. Related Background Art
In the prior art, as the electrophotographic method, there have been known a large number of methods as described in U.S. Pat. No. 2,297,691, Japanese Patent Publication No. 42-23910, Japanese Patent Publication No. 43-24748. Generally, the method comprises utilizing a photoconductive substance, forming electrical latent images on a photosensitive body by various means, subsequently developing the latent images by use of a toner, transferring the toner images onto a transfer material such as paper, etc. if necessary, then fixing the images by various methods to obtain a copied product. As the fixing method, there are the pressure fixing system of passing through two or more metal rolls, the oven fixing system of passing through the heated atmosphere heated by an electrothermal heater, and the currently most popular hot-roll fixing system by passing through heated rollers.
The pressure heating system using heated rollers performs fixing by passing the sheet to be fixed through the hot rollers under pressure, where the toner image on the sheet is in contact with the roller surface which is made of a toner-releasing material. According to this method, since the surface of the hot roller contacts the toner image on the sheet to be fixed under pressure, heat efficiency during fusion of the toner image onto the sheet to be fixed is extremely good and also fixing can be effected rapidly, and therefore it is very effective in a high speed electrophotographic copying machine. In the above-mentioned method, since the hot roller surface comes in contact with the toner image in melted state under pressure, a part of the toner image may adhere and transfer onto the surface of the fixing roller, which is retransferred onto the next sheet to be fixed to give rise to the so-called off-set phenomenon, thereby contaminating the sheet to be fixed. It is one of the essential conditions of the hot roller fixing system to avoid adhesion of the toner onto the hot fixing roller surface.
For obtaining a toner satisfying the above essential conditions, improvements of the toner binder have been attempted. For example, Japanese Patent Publication No. 1-15063 proposes a developer by use of a resin in which a polyester resin and two kinds of vinyl resins of different gel contents (80% or more and less than 10% of THF insolubles) are simply blended. Japanese Patent Application Laid-open No. 63-223662 proposes a developer improved in the anti-off-set property by controlling the content of the THF insolubles in the binder resin in the toner.
As the polymerization method to obtain a resin containing such amount of THF insolubles as mentioned above, the suspension polymerization or emulsion polymerization method may be preferably employed.
Since these polymerization methods are carried out generally in aqueous solutions, after completion of the reaction, there exists no removal step of the solvent and the residual polymerizable monomer from the solution polymerization method, and also the temperature of the reaction system is limited. A resin containing much THF insolubles is liable to have the monomers confined within the resin, whereby the content of the remaining monomer in the resin must be larger. When the amount of the remaining monomer is large, there exist problems such as (1) generation of odor during toner preparation, particularly during melting and kneading process, which degrades the working environment, or (2) generation of odor during image formation, giving unpleasant feelings. In recent years, recording methods using the electrophotographic method are expanding its application scope, so that now they are used in ordinary offices and homes. As to the developer, it has become necessary to pay attention not only to the safety of the substance, of course, but also to odor generation during fixing.
Japanese Patent Application Laid-open No. 55-155632 proposes the improvement of the anti-off-set property, storage property and fluidity by the use of a polymer as a binder resin for a developer, where the resin contains less than 0.1% by weight of the residual solvent or the polymerizable monomers used in the polymerization.
Japanese Patent Application Laid-open No. 53-17737 describes that remaining of the polymerizable monomers influences the triboelectric chargeability, blocking property and fixability of the developer. Further, as to the binder resin for developing, it has been proposed to reduce the remaining solvent or the polymerizable monomers used for obtaining the polymer.
Japanese Patent Application Laid-open No. 64-70765 proposes a resin for a developer with a remaining monomer content of 200 ppm or less by evaporating the water after polymerization, for overcoming such problems as degrading of the working environment particularly caused by the generation of odor during melting and kneading, or generation of odor during copying to give unpleasant sensations. In such literature, there is the description that antiblocking property and resistance to vinyl chloride plasticizer become low, or the problem of odor remains if the remaining monomer amount exceeds 200 ppm.
All of the toners obtained according to the methods as mentioned above were still not satisfactory in the improvement in both anti-off-set property and odor.
Particularly, in recent electrophotographic apparatus, for the prevention of ozone generation by high voltage application during formation of electrostatic latent images on the photosensitive body surface, contact charging means has been used in place of corona charging. As the result of substantial absence of ozone generation, in the electrophotographic apparatus by use of the contact charging means, it has become possible to eliminate an ozone filter, whereby the problem of odor of the developer becomes marked when no ozone filter is mounted.
On the other hand, in recent years, accompanying the speed-up of recording methods by the electrophotographic method, there have been developed various contrivances for improving fixability in the hot-roll fixing system. As the method for improving the binder resin, improvements have been made to lower the glass transition temperature, thereby enabling low temperature fixing of the developer. As the means for controlling the glass transition temperature (Tg) of the resin, the method of controlling the components of the resin may be employed. In the styrene-acrylic resin which has been used most generally as a binder resin, the method of increasing the ratio of the acrylic component in the resin has been employed. As the acrylic resin, one which has the significant effect of lowering the glass transition point (Tg) tends to be used frequently.
As described above, when the ratio of acrylic monomer of low reactivity as compared with styrene monomer is increased, the remaining amount of the monomer in the resin after completion of the polymerization will be also increased, whereby problems as mentioned above will be brought about. For this reason, resin synthesis is carried out by use of an acrylic component with higher reactivity. According to this method, although the amount of the remaining monomer in the resin is considerably reduced, a large amount of an acrylic monomer is required to lower the glass transition temperature (Tg). As a result, there is a tendency to reduce charge amount, resulting in lower developability, whereby the image density is lowered.